1. Field of the Invention
The present invention relates to a technology of an optical waveguide.
2. Description of the Related Art
An optical waveguide used in an optical modulator is manufactured by forming a metallic film of titanium (Ti) etc. on a part of a dielectric substrate (electro-optical crystal), such as an lithium niobate (LN: LiNbO3) substrate and a lithium tantalite (LiTaO2) substrate, and thermally diffusing the metallic film or by performing proton exchange in benzoic acid after patterning. After the optical waveguide is formed, electrodes are formed near the waveguide. By applying voltage to the electrodes, light modulation can be performed. Light transmitted through the waveguide is apt to be absorbed by the electrodes. To prevent such absorption, a buffer layer is formed between the LN substrate and the electrodes. A silicon oxide (SiO2) film of 0.2 to 1 micrometer (μm) thick is used as the buffer layer.
A linear optical waveguide and a bent optical waveguide are used in combination when an optical modulator is miniaturized, more than two optical modulators are connected in a multistage structure, or a long optical waveguide is formed in a single chip. Linear optical waveguides can be folded by bending an optical waveguide, thereby making a size compact.
FIG. 10A is a plan view of a conventional bent waveguide, and FIG. 10B is a cross-section of the conventional bent waveguide. An optical waveguide 502 bent at a radius of curvature R is formed on a substrate (LN substrate) 500 at a bent portion. The optical waveguide 502 is formed by diffusing an impurity such as Ti on the substrate 500. At the bent portion of the optical waveguide 502, light loss is large. Therefore, a groove 505 is formed at the outside of bend at the bent portion of the optical waveguide 502 on the substrate 500 so that light loss due to radiation is suppressed. A buffer layer 503 is formed on the substrate 500 as shown.
The formation of the groove 505, however, cannot eliminate light loss completely. The smaller the radius of curvature R is, the larger the loss becomes, and such large loss cannot be neglected. Light loss due to radiation can be reduced by enhancing the confinement of light within the optical waveguide 502.
FIG. 11 is a schematic of a conventional optical waveguide with enhanced confinement of light. An impurity diffusion layer 602 that is formed, by diffusing an impurity having a low refractive index, such as magnesium oxide (MgO), underneath a convex (ridge-shaped) optical waveguide 601 formed on a substrate 600 as shown in FIG. 11 or around the optical waveguide 601 (not shown) so as to lower a refractive index of the optical waveguide (for example, Japanese Patent Application Laid-Open Publication No. H01-201609). In such structure, the optical waveguide 601 functions as a core while the impurity diffusion layer 602 functions as a cladding. A large difference in refractive index between the core and the cladding enhances the confinement of light in the optical waveguide 601, thereby reducing light loss. Other examples of structures to enhance the confinement of light include a structure in which a diffusion layer formed with an impurity having a low refractive index is arranged along a bent portion of an optical waveguide at the outside of the bend (for example, Japanese Patent Application Laid-Open Publication No. S63-157109), and a structure in which a diffusion layer formed with an impurity having a low refractive index is arranged between optical waveguides (for example, Japanese Patent Application Laid-Open Publication No. H03-148625).
The conventional technique shown in Japanese Patent Application Laid-Open Publication No. H01-201609 (FIG. 11), however, is intended for a linear optical waveguide, and if merely the structure shown in FIG. 11 is applied to a bent waveguide, light loss increases due to radiation at a bent portion. In a manufacturing process, grooves are formed at both sides of the optical waveguide 601 to form a ridge portion where the optical waveguide 601 is formed into a convex shape. The grooves are formed by etching the substrate 600. If an etching pattern shifts with respective to the bend pattern of the optical waveguide 601 (shift in patterns), the impurity (MgO) having a low refractive index at the inside of the bend pushes a light mode to the outside. As a result, light loss increases.
Moreover, when the impurity (MgO) is formed over the entire surface of the substrate in a pattern of a greater width, as in the case of Japanese Patent Application Laid-Open Publication No. H01-201609, diffusion of Ti becomes difficult. This causes increase in light loss, or fluctuation of characteristics due to stress to the impurity generated when temperature changes. For example, an operating point voltage (bias voltage) of an optical modulator becomes unstable.
Furthermore, in the techniques described in Japanese Patent Application Laid-Open Publication Nos. S63-157109 and H03-148625, if a pattern of Ti and a pattern of MgO overlap each other, diffusion is hindered at the overlapping portion to be a nontransparent portion. As a result, light loss rather increases.